Pump-coupled micromasers: coherent and incoherent coupling

Abstract

We consider two micromasers coupled via the pumping beam of initially excited two-level atoms traversing the two cavities in a sequence. The atomic populations and coherences pumping the second micromaser are, therefore, prepared by the first one. To separate the effects of atomic coherence we compare the two cases of incoherent and coherent coupling, i.e., when the states of the atoms are measured between the cavities (`which path') and when not. Using an exact solution for the incoherent case, we find that the second micromaser undergoes abrupt transitions between distinct phases of the photon statistics triggered, via injected absorption, by the first one. In the case of coherent coupling, we consider the exact master equation of the fields first and, after finding its nonlinear expansion, apply standard Fokker–Planck techniques. Complementing the nonlinear treatment, we also employ numerical simulations to investigate a different regime of the system. In addition to the incoherent effects, we find that atomic coherence significantly modifies the mean and the variance of the photon number of the second field and, at the same time, establishes first order (phase locking) and second order correlations between the two fields at steady state. The time evolution exhibits abrupt jumps in the locking of the relative phase between zero and π accompanied by shut-offs of the second micromaser. The coherence effects are particularly important in the region of small pumping parameters.